The sinus node is known as the heart's “natural pacemaker”. The sinus node sends out electrical signals to the lower chambers of the heart (ventricals), causing the chambers to contract at a steady rhythm of about 60 to 100 beats per minute.
When the heart is not beating at a steady rate, the irregular heartbeats are called arrhythmias. The most common type of sustained arrhythmia is atrial fibrillation. Atrial fibrillation (AF) is a rapid, irregular heart rhythm caused by abnormal electrical signals from the upper chambers of the heart (atrium). AF may increase the heart rate to and in excess of 100 to 175 beats per minute. As a result, the atria quiver rather than contracting normally, which can result in blood pooling in the atria, the formation of blood clots and strokes.
Recent studies suggest that the pulmonary veins (PV) are a major source of paroxysmal atrial fibrillation (AF) (Haissaguerre M, Jais P. Shah DC, Takahashi A, Hocini M, Quiniou G. Garrigue S. Le Mouroux A, Le Metayer P. Clementy J. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. N Eng J Med 1998;339:659–66).
One method for treating AF is to burn out, freeze or surgically excise the tissues and pathways from which the abnormal signals arise (ablation).
Radiofrequency (RF) catheter ablation of arrythmogenic sites inside the pulmonary veins (FIG. 1a) has proven successful in eliminating recurrence of AF. However, difficulty in locating arrythmogenic foci and complications during RF ablation such as thrombus formation and pulmonary vein stenosis have limited the efficacy of this procedure. (Robbins IM, Colvin EV, Doyle TP, Kemp WE, Loyd JE, McMahon WS, Kay GN. Pulmonary vein stenosis after catheter ablation of atrial fibrillation. Circulation 1998;98:1769–1775; Chen SA, Hsieh MH, Tai CT, Tsai CF, Prakash VS, Yu WC, Hsu TL, Ding YA, Chang MS. Initiation of atrial fibrillation by ectopic beats originating from the pulmonary veins: electrophysiological characteristics, pharmacological responses, and effects of radiotrequency ablation. Circulation 1999;100: 1879–86; Shah DC, Haissaguerre M, Jais P. Catheter ablation of pulmonary vein foci for atrial fibrillation: PV foci ablation for atrial fibrillation. Thorac Cardiovasc Surg 1999;47 (Suppl 3):352–6; Hsieh MH, Chen SA, Tai CT, Tsai CF, Prakash VS, Yu WC, Liu CC, Ding YA, Chang MS. Double multielectrode mapping catheters facilitate radioirequency catheter ablation of focal atrial fibrillation originating from pulmonary veins. J Cardiovasc Electrophysiol 1999; 10:136–44).
For example, Shah et al. reported a 69% success rate in 110 patients, with 4% of the pulmonary veins showing stenosis after 8±4 months. (Shah DC, Haissaguerre M, Jais P. Catheter ablation of pulmonary vein foci for atrial fibrillation: PV foci ablation for atrial fibrillation. Thorac Cardiovasc Surg 1999;47 (Suppl 3):352–6). Chen et al. reported similar success rates of 86% in 68 patients, but with a much higher frequency (42%) of pulmonary vein stenosis after 6±2 months. (Chen SA, Hsieh MH, Tai CT, Tsai CF, Prakash VS, Yu WC, Hsu TL, Ding YA, Chang MS. Initiation of atrial fibrillation by ectopic beats originating from the pulmonary veins:electrophysiological characteristics, pharmacological responses, and effects of radiotrequency ablation. Circulation 1999;100: 1879–86). In a case study involving 18 patients, Robbins, et al. found pulmonary stenosis with severe pulmonary hypertension in 2 patients (11%) after 3 months, and concluded that the use of standard catheter technology for RF ablation of the pulmonary veins should be avoided. (Robbins IM, Colvin EV, Doyle TP, Kemp WE, Loyd JE, McMahon WS, Kay GN. Pulmonary vein stenosis after catheter ablation of atrial fibrillation. Circulation 1998;98: 1769–1775).
Another limitation of RF focal ablation is its inability to produce conduction block in a single application. Frequently, it is difficult to locate and effectively ablate the arrythmogenic foci. For example, Haissaguerre et al. reported the need for 5±5 RF ablation applications of from 60–120 seconds for each focus (Haissaguerre M, Jais P. Shah DC, Takahashi A, Hocini M, Quiniou G. Garrigue S. Le Mouroux A, Le Metayer P. Clementy J. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins. N Eng J Med 1998;339:659–66), while Chen et al. reported the need for 7±3 RF ablation applications of from 20–40 seconds. (Chen SA, Hsieh MH, Tai CT, Tsai CF, Prakash VS, Yu WC, Hsu TL, Ding YA, Chang MS. Initiation of atrial fibrillation by ectopic beats originating from the pulmonary veins: electrophysiological characteristics, pharmacological responses, and effects of radiotrequency ablation.Circulation 1999;100: 1879–86).
The limitations of RF focal ablation (FIG. 1a) have elicited the exploration of alternative ablation strategies.
One alternative approach is to electrically isolate the pulmonary veins from the left atrium by creating a circumferential line of conduction block at the PV ostia. RF energy has been used to create a contiguous line of conduction block from a series of discrete circular RF lesions, or 'spot welds' (FIG. 1b) (Schwartzman D. Circumferential radiobrequency ablation of pulmonary vein orifices: feasibility of a new technique. PACE 1999;22:711). However, spot welding may be tedious and ineffective because the creation of a contiguous circumferential lesion from a series of precisely placed individual RF lesions is difficult to achieve under the current imaging limitations of x-ray fluoroscopy. Furthermore, even in cases when good electrode-tissue contact can be achieved, RF heating produces superficial direct heating due to a rapid attenuation of the electrical current density with tissue depth, resulting in a steep temperature gradient in the subsurface tissue layers. The combination of poor electrode-tissue contact due to imaging limitations and steep temperature gradients may increase the probability of complications such as tissue vaporization, endothelial disruption, and coagulum formation. Endothelial disruption may be responsible for incidents of pulmonary vein stenosis which have been reported during previous pulmonary vain ablation studies using RF energy. (Robbins IM, Colvin EV, Doyle TP, Kemp WE, Loyd JE, McMahon WS, Kay GN. Pulmonary vein stenosis after catheter ablation of atrial fibrillation. Circulation 1998;98:1769–1775; Chen SA, Hsieh MH, Tai CT, Tsai CF, Prakash VS, Yu WC, Hsu TL, Ding YA, Chang MS. Initiation of atrial fibrillation by ectopic beats originating from the pulmonary veins: electrophysiological characteristics, pharmacological responses, and effects of radiotrequency ablation. Circulation 1999;100: 1879–86; Shah DC, Haissaguerre M, Jais P. Catheter ablation. of pulmonary vein foci for atrial fibrillation: PV foci ablation for atrial fibrillation. Thorac Cardiovasc Surg 1999;47 (Suppl 3):352–6; Hsieh MH, Chen SA, Tai CT, Tsai CF, Prakash VS, Yu WC, Liu CC, Ding YA, Chang MS. Double multielectrode mapping catheters facilitate radioirequency catheter ablation of focal atrial fibrillation originating from pulmonary veins. J Cardiovasc Electrophysiol 1999; 10: 136–44).